Absolute-scale measurement of compton backscattering in germanium at 105.3 keV

The differential cross section d²σ/dEdΩ was measured at an incident photon energy of 105.3 keV in germanium on absolute scale. The measurements were made using two high purity Ge detectors that operated in a coincidence mode. One detector served as the target and detector of ejected electrons, and another as the detector of Compton-scattered photons. It was found that fast (multiple) cascades have little influence on the data of the 105.3 keV crossover transition. An improved determination of the detector efficiency was made. The calculated values of the cross-section d²σ/dEdΩ using the impulse approximation (IA) with Hatree–Fock (HF) wave functions are in excellent agreement with the experimental cross-sections, while the IA with hydrogen like (HL) wave functions give fair agreement. A comparison of the calculated values of the cross-sections obtained with HF and HL wave functions for each subshell in germanium gives new values for effective charges, which improve agreement between the results of IA with HL and experimental values.     

Analytical computation of the Compton continuum in gamma-ray spectrometry

In gamma-ray spectrometry with solid state detectors if the photons have only one collision in the detector, the peak to total ratio should be equal to the ratio of photoelectric to total cross sections. However, it is well known that it is higher and increases with the detector size for all energies due to multiple collisions in the detector. With the data on the difference between the two ratios we estimate the higher order collisions and use this estimate to compute the counts distribution in the Compton continuum. The method is simple and essentially physics oriented without recourse to any empirical fitting of the spectral data. Results compare remarkably well with the experimental observations of Compton continuum.     

New method to measure the K-shell absorption jump factor in some rare-earth elements

The Compton scattering of 59.54 keV gamma rays by an Al scatterer has been used as a primer source at scattering angles from 48 to 118° by using a Si(Li) detector, and this primer gamma ray has been send to absorbers including Gd, Tb, Dy, Ho and Er. A new method has been developed to determine the K-shell absorption jump factor of elements and compounds. This method is based on simultaneous measurement of fluorescence radiation and scattered radiation, thus avoiding the problems with measuring the source strength and source-to-detector solid angle. In this method, the jump factor is effected from the scattering angle. Evident energies near to K-absorption edges of each lanthanide element have been determined for chosen angles, after the incident photon energy (59.5 keV) is exposed to Compton scattering from Al (secondary source). The experimental absorption jump factors are compared with the theoretical estimates and literature experimental values.     

Need for further inelastic scattering measurements at X-ray energies

With recent improvements in both theory and experiment for scattering of X-rays from atoms, it is possible now to make a more quantitative comparison, and see whether agreement is being obtained within the much more stringent limits set by the present calculations and measurements. Comparing with present theory, measured whole atom Compton scattering cross sections in the photon energy range 11–40 keV using synchrotron X-ray sources demonstrate that a dramatic improvement in the precision of scattering measurements has been achieved. However, circumstances are also identified in which further experimental data is needed in order to test the adequacy of present theoretical approaches.     

Experimental investigation of multiple scattering of 662 keV gamma rays in zinc at 90°

Compton scattering investigations usually examine the case when photon has undergone only one Compton collision in the sample. The probability of a photon being scattered several times may be significant for a target of finite dimensions both in depth and lateral dimensions. The present experiment is undertaken to study the intensity and energy distributions of 662 keV gamma rays multiply scattered from a zinc target of various thicknesses at a scattering angle of 90° with the scattered photons being detected by an HPGe gamma detector. We observe that with an increase in target thickness, the number of multiply scattered photons also increases and saturates at a particular value of the target thickness (saturation depth). This supports the work of Paramesh, L., Venkataramaih, L., Gopala, K., Sanjeevaih, H. [1983. Z-dependence of saturation depth for multiple scattering of 662 keV photons from thick samples. Nucl. Instrum. Methods 206, 327–230]. The double Compton scattered peak is also observed in the experimental spectra, with a position in agreement with the predictions of Fernandez, J.E. [1991. Compton and Rayleigh double scattering of unpolarized radiation. Phys. Rev. A44, 4232–4248] and Barnea, G., Dick. C.E., Ginzburg. A.E., Seltzer. S.M. [1995. A study of multiple scattering background in Compton scatter imaging. NDT E Int. 28. 155–162].     

Compton suppression system at Penn State Radiation Science and Engineering Center

A Compton suppression system is used to reduce the contribution of scattered gamma-rays that originate within the HPGe detector to the gamma-ray spectrum. The HPGe detector is surrounded by an assembly of guard detectors, usually NaI(T1). The HPGe and NaI(T1) detectors are operated in anti-coincidence mode. The NaI(T1) guard detector detects the photons that Compton scatter within, and subsequently escape from the HPGe detector. Since these photons are correlated with the partial energy deposition within the detector, much of the resulting Compton continuum can be subtracted from the spectrum reducing the unwanted background in gamma-ray spectra. A commercially available Compton suppression spectrometer (CSS) was purchased from Canberra Industries and tested at the Radiation Science and Engineering Center at Penn State University. The PSU-CSS includes a reverse bias HPGe detector, four annulus NaI(T1) detectors, a NaI(T1) plug detector, detector shields, data acquisition electronics, and a data processing computer. The HPGe detector is n-type with 54% relative efficiency. The guard detectors form an annulus with 9-inch diameter and 9-inch height, and have a plug detector that goes into/out of the annulus with the help of a special lift apparatus to raise/lower. The detector assembly is placed in a shielding cave. State-of-the-art electronics and software are used. The system was tested using standard sources, neutron activated NIST SRM sample and Dendrochronologically Dated Tree Ring samples. The PSU-CSS dramatically improved the peak-to-Compton ratio, up to 1000:1 for the ¹³⁷Cs source.     

An innovative Compton suppression spectrometer and its capability of evaluation in multi-elemental neutron activation analysis

A new Compton suppression system was established. A low energy HPGe detector (LO-AX) is used as the analyzing detector. It is completely shielded by a 50%n-type HPGe and two NaI(TI) detectors. Experiments show that a substantial Compton suppression effect is also achieved in the low energy region down to 15 keV. The capability of applying this system in INAA was evaluated for 13 elements (As, Cd, Co, Cr, Hg, Mo, Ni, Sb, Se, Sr, Th, U and Zn) in biological and environmental samples. The benefits and problems of using this Compton suppression spectrometer in INAA are examined for each element.     

At-edge minima in elastic photon scattering amplitudes for dilute aqueous ions

Elastic photon scattering and absorption in the vicinity of core atomic orbital energies give rise to resonances in the elastic photon scattering cross-section. Of interest is whether a dilute-ion aqueous system provides an environment suitable for testing independent particle approximation (IPA) predictions. Predictions of the energy of these resonances have been determined for a Dirac–Slater exchange potential with a Latter tail. At BM28 (ESRF), tuneable X-rays were obtained at eV resolution using a 1 1 1 Si monochromator. From target systems including Cu²⁺ and Zn²⁺, the X-rays were scattered through high angle from an aqueous medium contained in a thin Perspex cell provided with 8 μm kaplan windows. An energy resolution of ∼500 eV from the HPGe detector was adequate to separate the elastic scattering signal from K_α radiation but not from Compton or K_β contributions. The Compton contribution from the medium was removed assuming validity of the relativistic impulse approximation. The contribution due to K_β fluorescence and the resonant X-ray Raman scattering process were handled by assuming the branching ratio for K_α and K_β contributions to be constant and to be accurately described by fluorescent yields measured above edge. At ionic concentrations ranging from 0.01 to 0.1 mol/l, resonance structures accord with predictions of elastic scattering cross-sections calculated within IPA. Amplitudes calculated using modified form-factors and anomalous scatter factors computed from a Dirac–Slater exchange potential were convolved with a Lorentzian of several eV (FWHM).     

Monte Carlo simulation of the backscatter region in photon-excited energy dispersive X-ray fluorescence spectra

The backscatter region of energy-dispersive X-ray fluorescence spectra obtained by a typical radioisotope system is examined in detail. A Monte Carlo simulation program which incorporates all the information on photon scattering processes including electron momentum distributions in target atoms, form factors and scattering factors for the cross-sections is presented. The program uses extensive variance reduction techniques and has the option of simulating any type of either single or any specified combination of multiple scatters or the complete process as appears in a real detector response spectrum. Based on several simulated spectra various conclusions are reached including that the energy spread of Compton peaks depends mainly on the broadening effect of electron momenta together with detection system resolution, and that the intensity of double scatters is roughly an order of magnitude lower than that of single scatters for thick targets.     

Some experiments on γ-scattering between 122 and 145 keV at small angles

Differential cross section for scattering of 145.4 keV gamma rays by B, C, Al, Cu and Cd have been measured from 5–25°. For angles <10° it was not possible to separate Rayleigh and Compton scattering; therefore, the sum of the cross sections is given. Rayleigh cross sections have been measured for Pb at 122.1 and 136.5 keV at angles between 20° and 70°. The experimental results are compared with the form factor theory for Rayleigh scattering and the incoherent scattering factor theory for Compton scattering.     

Elastic scattering of gamma rays and X-rays

Studies of elastic gamma ray scattering were pursued independently by the groups of Prof. Ghose and the author for several decades in spite of somewhat meagre support. Several techniques for such studies developed by the two groups and some of the results obtained in the energy range from tens of keV to about 1.5 MeV are described briefly. The theoretical background necessary for understanding these results is also outlined. The results showed the importance of Modified Relativistic Form Factor (MRFF) approximation in the explanation of atomic Rayleigh scattering cross sections in the small angle regime and the necessity for an inclusion of real Delbrück scattering amplitudes at large scattering angles. Dispersion corrections to form factor amplitudes or the so-called anomalous scattering factors are shown to be needed at photon energies close to electron binding energy thresholds. A few promising future extensions of such studies are indicated at the end.     

The correction vector method for three-photon absorption: the effects of pi conjugation in extended rylenebis(dicarboximide)s

A correction vector method within the multireference determinant single and double configuration interaction approximation coupled with the semiempirical intermediate neglect of differential overlap Hamiltonian has been developed for the computation of single and multiphoton absorption spectra of conjugated molecules. We study the effect of pi conjugation on these properties in the extended rylenebis(dicarboximide)s. The one-, two-, and three-photon absorption cross sections of the lowest-lying excited states show a power law dependence on the conjugation length, with exponents of about 1.3, 2.6, and 5.6, respectively. The maximum value of the three-photon absorption cross section in these molecules is calculated to be 1.06x10(-78) cm6 s2photon2 for photon energy at 0.57 eV.     

Two‐Photon Absorption Cross‐Sections of Reference Dyes: A Critical Examination

The electronic structure and one‐ and two‐photon absorption spectra of four fluorophores, p‐bis(o‐methoxystyryl)benzene (Bis‐MSB), coumarin 307, fluorescein and rhodamine B, commonly used as reference compounds for two‐photon absorption spectra, have been theoretically calculated and compared with available experimental data. The possible reasons for the wide discrepancies in two‐photon absorption cross‐sections reported in the literature are discussed on the basis of the theoretical findings. The role of a solvent environment on the electronic one‐ and two‐photon absorption spectra is also studied. We highlight some necessary precautions that one needs to take when comparing literature results of two‐photon absorption cross‐sections.     

Corrections to the impulse approximation for use in electron compton scattering experiments

The breakdown of the impulse approximation for K-shell electrons in electron Compton scattering measurements on solids is parameterized in a form which makes it possible to assess the importance of this effect and where necessary to apply a suitable correction.     

Total photon cross sections for low energy photons in light elements

A large amount of compiled data on total photon cross sections is available in the literature. However, for some elements, in the low Z region data is sparse, although new theoretical data have become available. In the present study, transmission experiments in the low photon energy region 5–60 keV were carried out for four low Z elements using a Freolectric cooled Si(Li) detector attached to a System-100 multichannel analyser. Results are presented in the light of present theory.     

A dual purpose Compton suppression spectrometer

A gamma-ray spectrometer with a passive and an active shield is described. It consists of a HPGe coaxial detector of 42% efficiency and 4 NaI(Tl) detectors. The energy output pulses of the Ge detector are delivered into the 3 spectrometry chains giving the normal, anti- and coincidence spectra. From the spectra of a number of ¹³⁷Cs and ⁶⁰Co sources a Compton suppression factor, SF and a Compton reduction factor, RF, as the parameters characterizing the system performance, were calculated as a function of energy and source activity and compared with those given in literature. The natural background is reduced about 8 times in the anticoincidence mode of operation, compared to the normal spectrum which results in decreasing the detection limits for non-coincident gamma-rays up to a factor of 3. In the presence of other gamma-ray activities, in the range from 5 to 11 kBq, non- and coincident, the detection limits can be decreased for some nuclides by a factor of 3 to 5.7.     

Shape resonances and partial photoemission cross sections of solid SF6 and CCl4

Photoelectron energy distribution curves from solid films of SF₆ and CCl₄ have been measured in the photon energy range 10 ? hr ? 40 eV using synchrotron radiation. The binding energies, peak widths and relative partial cross sections have been determined. In the photoelectron spectra a 1:1 correspondence to the gas phase is observed for the occupied molecular orbitals, and a straightforward assignment of the occupied valence bands emerges. Furthermore, the cross sections of the individual orbitals show for both samples great similarities to the gas phase. For SF₆ detailed structures are visible in the cross sections which are only partly interpreted as shape resonances. A new assignment for the 6t_1u shape resonance is proposed and the resonance energies are related to X-ray absorption and electron scattering data. Furthermore a comparison of the total photoemission cross section to the optical reflection spectrum of solid SF₆ is presented. For CCl₄ less structures are ob served in the partial cross sections. They are all interpreted as shape resonances. An energetic scheme of the virtual orbitals is proposed for CCl₄.     

Monte Carlo simulation of X-ray fluorescence spectra: Part 4. Photon scattering at high X-ray energies

The photon scattering model of a Monte Carlo simulation code for synchrotron radiation X-ray fluorescence (SRXRF) spectrometers is evaluated at high X-ray energies (60–100 keV) by means of a series of validation experiments performed at Beamline BW5 of HASYLAB. Using monochromatic X-rays, Compton/Rayleigh multiple scattering experiments were performed on polypropylene, Al and Cu samples. Especially in the case of the first two matrices multiple Compton scattering occurs with high probability. This work demonstrates that the simulation model provides a reliable estimate of the spectral distribution of the multiply scattered linearly polarized photon beam as observed by an HPGe detector. Next to variations in sample composition and thickness, the ability of the code to simulate various detection geometries has also been verified. As an application of the code, the achievable detection limits of SRXRF for rare earth elements as obtained with white beam and monochromatic (80 keV) excitation are compared.